Improve source position in parsing warnings

A mere space error:
```
...
... some text {.class}
```

Yields a warning:

```
{
message = "Ignoring unattached attribute",
pos = 30
}
```

Where `pos` is the byte position.
When the sourcepos map is enabled, it would be handy to have the usual `line:col:charpos` in warnings, as we can have on nodes, for easier investigation by the typist.